FIG. 1 is an exploded view of an example of a miniature camera with fixed focal length.
This type of camera 1 comprises an image sensor 10 arranged on a substrate 11, an optical block 20 comprising several lenses 20a, 20b, 20c assembled in a barrel 21, and a mount 22 with which the substrate 11 and the barrel 21 are assembled.
After such a camera leaves manufacturing, a focusing operation is generally necessary for objects located at infinity and captured by the camera to be clear.
This focusing operation consists of moving the barrel 21 containing the lenses relative to the image sensor 10 and making acquisitions of images of an object (typically, a sight) place at infinity or at least at a sufficiently large distance from the camera.
Displacement of the barrel is generally ensured by rotation of said barrel in the mount by cooperation of a thread 210 of the barrel and an inner thread 220 of the mount.
Analysis of images during this displacement determines the position of the barrel corresponding to the maximum sharpness of the image.
The barrel 21 is then fixed in this position in the mount 22, for example by way of local welding at the junction between the mount 22 and the barrel 21.
Other cameras comprise a function called “autofocus” which consists of automatically ensuring the sharpness of a captured object, whether lying at infinity or near the camera.
To ensure this function, a current solution is to integrate an optical device with variable focal length at the front of the camera, that is, upstream of the fixed optical block on the path of light towards the image sensor.
Among optical devices which can be used for this purpose, devices comprising a deformable membrane can be selected advantageously. Documents FR2919073, FR2950154 and FR2950153 describe examples of such devices with deformable membrane. These devices comprise generally: at least one deformable membrane; a support to which a peripheral anchoring area of said membrane is connected; a cavity filled with a constant volume of fluid, said cavity being delimited in part by said membrane; an actuation device of a area of the membrane located between the peripheral anchoring area and a central part of the membrane, configured to bend by application of electrical actuation voltage so as to displace some of the fluid volume towards the centre or the periphery of the cavity.
FIG. 2 is an exploded view of an autofocus camera 1 integrating an optical device with variable focal length 30. The elements designated by the same reference numerals as in FIG. 1 fulfil the same function and are therefore not described again.
The optical device with variable focal length comprises an optical area whereof the focal length is adjustable and a device for electrical connection to the substrate.
In general, focusing at infinity is executed before integration of the optical device with variable focal length. In fact, once the optical device with variable focal length 30 is put into place, it is no longer possible to access the barrel 21 containing the fixed optics to move it due to the presence, in front of the barrel 21, of the optical device with variable focal length 30 and the connection 31 of the latter with the substrate 11, which prevents rotation of the barrel 21 in the mount 22.
There are two processes for performing focusing of the camera before integration of the optical device with variable focal length.
A first process is based on the same principle as for cameras with fixed focal length, specifically displacement of the barrel comprising the optical block relative to the mount so as to ensure the sharpness of the image of an object located at infinity. In this case, the optical device with variable focal length must have zero optical power (0 dioptre) to maintain focusing of objects located at infinity. To focus an object located near the camera, positive optical power of the order of 10 dioptres is typically required. The range of optical power which the optical device with variable focal length must be able to ensure therefore comprises at least the range between 0 and 10 dioptres.
Even though less well known, it is also possible to conduct focusing at a finite distance, for example 20 cm. In this case, the optical device with variable focal length must have zero optical power (0 dioptre) to maintain focusing objects located at this distance of 20 cm. In this example, to focus on an object located 10 cm from the camera, a positive optical power of the order of 5 dioptres is typically required. An object located at infinity is clear for an optical power of −5 dioptres. The range of optical power which must be ensured by the optical device with variable focal length therefore comprises at least the range between −5 and +5 dioptres. As compared to focusing at infinity, focusing at 20 cm therefore brings in offset of −5 dioptres of the operating range of the optical device with variable focal length.
Some optical devices can see their optical power vary as a function of the service temperature of the camera. Also, the manufacturing can induce dispersions of optical power.
Consequently, to ensure coverage of the required range of optical power, it can be advantageous or even necessary to ensure that the optical device with variable focal length is slightly divergent after manufacturing, for example of the order of −1 to −5 dioptres, or even −10 dioptres in some cases. From a nominal divergent configuration, transition at 0 dioptre and accordingly focusing at infinity are ensured. However, near focusing, needing around 10 dioptres of optical power, is difficult to attain and needs an even greater variation in optical power since the rest position is divergent: a variation of 11 dioptres for an initial position of −1 dioptre, a variation of 15 dioptres for an initial position of −5 dioptres, etc.). This operating principle always needs actuation of the device with variable focal length, whether performing focusing at infinity or near, for example at 20 cm as in the example developed above, and irrespective of the range of operation of the optical device with variable focal length (for example [0; 10] dioptres or [−5; +5] dioptres as in the cases disclosed above).
FIG. 3A illustrates an example of variation in optical power of a device with variable focal length for an autofocus camera focusing according to this first process. In this example, consideration is given to a linear variation of the optical power P (in dioptres) with the electrical voltage U (in volts) applied to the actuation device of the optical device with variable focal length. The optical power at rest (that is, without application of electrical voltage) is −5 dioptres in this example. This diagram shows two ranges of actuation of the device:                a first range (hatching rising from the left to the right) between −5 and 0 dioptres aims merely to compensate the variations in optical power due to manufacturing dispersions and/or variations in temperature.        a second range (hatching descending from left to right) between 0 and 10 dioptres ensures focusing at infinity and near the camera.        
The first 3 volts applied to produce zero optical power are not useful for focusing and can therefore be considered as lost; focusing is done in the range of electrical voltage between 3 and 10 V.
In a second focusing process, as in the preceding process the barrel comprising the optical block is moved relative to the mount to ensure sharpness of the image of an object located at infinity; however, once focusing at infinity is carried out, defocusing is purposely introduced by displacement of the barrel relative to its optimal position if the optical power of the device with variable focal length is not zero. Alternatively, instead of focusing at infinity then defocusing, the optical device can be focused directly at a smaller distance. If the optical power of the optical device with variable focal length is zero, the latter must be focused at infinity. This particular focusing is conceived so as to exactly compensate the initial optical power of the device with variable focal length which is then integrated into the camera. In other terms, the ensemble of the optical block and the device with variable focal length at rest must ensure focusing at infinity, actuation being necessary only to ensure near focusing.
FIG. 3B illustrates an example of variation in optical power of the same device with variable focal length as that of FIG. 3A for an autofocus camera focusing according to this second process. The optical power at rest (that is, without application of electrical voltage) is −5 dioptres, which is compensated by defocusing of the fixed optics of the camera by 5 dioptres or direct focusing on an object located 20 cm from the camera. In this case, variation in optical power of 10 dioptres (from −5 to 5 dioptres) is therefore sufficient to carry out focusing at infinity and near. This corresponds to a range of electrical actuation voltage of between 0 and 7 V.
As compared to the first focusing process, this second process has the advantage that the whole range of variation in optical power (and therefore the range of electrical actuation voltage) is used for focusing. Also, the range of electrical actuation voltage is reduced. However, a major disadvantage of this process is that the dispersion after manufacturing of devices with variable focal length must be minimal and well controlled, which is highly restrictive. On the other hand, in the absence of a range of operation for compensating the effects of temperature on the focal length at rest of the device, the service temperature of the camera is likely to directly affect focusing of the camera.